[webkit-dev] New web-facing canvas feature: "opaque" attribute

[Gregg Tavares]

On Thu, Mar 14, 2013 at 1:38 PM, Ryosuke Niwa <rniwa at webkit.org> wrote:

> On Thu, Mar 14, 2013 at 12:55 PM, Dean Jackson <dino at apple.com> wrote:
>
>>
>> On 15/03/2013, at 6:50 AM, Dana Jansens <danakj at chromium.org> wrote:
>>
>> On Thu, Mar 14, 2013 at 3:46 PM, Dean Jackson <dino at apple.com> wrote:
>>
>>> I'm not sure I like this proposal. Why is canvas special? Why doesn't
>>> <img> get an opaque attribute (or flag)? Why not every element?
>>>
>>
>> There is ongoing work to infer opaqueness in every other kind of element
>> when possible. See for example
>> https://bugs.webkit.org/show_bug.cgi?id=70634
>>
>>
>> Yes, I'd prefer to infer it rather than specify it. For example, I could
>> infer that a canvas is opaque if it has a non-transparent CSS
>> background-color.
>>
>
The content of the canvas has to be blended with the background color so
that doesn't help optimization. If there's a background color you first
have to do a full blend of the contents of the canvas with the background
color. Where as if the canvas has no alpha then that step can be avoided.



>
> I like this approach. It means that developers don't have to explicitly
> use this feature to get the performance benefits.
>
> In fact, this is the preferred performance optimization approach on the
> Web. We don't provide explicit APIs to optimize performance. We make
> sensible APIs which allows us to implement more optimizations on common
> cases behind the scene.
>
> - R. Niwa
>
>
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